ADL5387ACPZ-R71 datasheet(19/28 Pages) AD

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ADL5387

Rev. 0 | Page 19 of 28

As an example, a second-order, Butterworth, low-pass filter

design is shown in Figure 56 where the differential load impedance

is 500 Ω, and the source impedance of the ADL5387 is 50 Ω.

The normalized series inductor value for the 10-to-1, load-to-

source impedance ratio is 0.074 H, and the normalized shunt

capacitor is 14.814 F. For a 10.9 MHz cutoff frequency, the

single-ended equivalent circuit consists of a 0.54 μH series

inductor followed by a 433 pF shunt capacitor.

The balanced configuration is realized as the 0.54 μH inductor

is split in half to realize the network shown in Figure 56.

433pF

RS = 50Ω

RL= 500Ω

0.54µH

0.27µH

433pF

BALANCED

CONFIGURATION

DENORMALIZED

SINGLE-ENDED

EQUIVALENT

RS = 50Ω

= 0.1

RL= 500Ω

LN = 0.074H

14.814F

NORMALIZED

SINGLE-ENDED

CONFIGURATION

= 25Ω

= 250Ω

fC = 10.9MHz

fC = 1Hz

Figure 56. Second-Order, Butterworth, Low-Pass Filter Design Example

A complete design example is shown in Figure 59. A sixth-order

Butterworth differential filter having a 1.9 MHz corner frequency

interfaces the output of the ADL5387 to that of an ADC input.

The 500 Ω load resistor defines the input impedance of the

ADC. The filter adheres to typical direct conversion WCDMA

applications, where 1.92 MHz away from the carrier IF frequency,

1 dB of rejection is desired and 2.7 MHz away 10 dB of rejection

is desired.

Figure 57 and Figure 58 show the measured frequency response

and group delay of the filter.

–20

–15

–10

–5

3.0

2.5

2.0

1.5

1.0

0.5

FREQUENCY (MHz)

Figure 57. Baseband Filter Response

900

800

700

600

500

400

300

200

100

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

FREQUENCY (MHz)

Figure 58. Baseband Filter Group Delay

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